on the heavens-第15部分

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but　so　as　to　make　equal　angles；　and　thus　to　a　single　centre；　that　of　the　earth。　It　is　clear；　then；　that　the　earth　must　be　at　the　centre　and　immovable；　not　only　for　the　reasons　already　given；　but　also　because　heavy　bodies　forcibly　thrown　quite　straight　upward　return　to　the　point　from　which　they　started；　even　if　they　are　thrown　to　an　infinite　distance。　From　these　considerations　then　it　is　clear　that　the　earth　does　not　move　and　does　not　lie　elsewhere　than　at　the　centre。　　　From　what　we　have　said　the　explanation　of　the　earth's　immobility　is　also　apparent。　If　it　is　the　nature　of　earth；　as　observation　shows；　to　move　from　any　point　to　the　centre；　as　of　fire　contrariwise　to　move　from　the　centre　to　the　extremity；　it　is　impossible　that　any　portion　of　earth　should　move　away　from　the　centre　except　by　constraint。　For　a　single　thing　has　a　single　movement；　and　a　simple　thing　a　simple：　contrary　movements　cannot　belong　to　the　same　thing；　and　movement　away　from　the　centre　is　the　contrary　of　movement　to　it。　If　then　no　portion　of　earth　can　move　away　from　the　centre；　obviously　still　less　can　the　earth　as　a　whole　so　move。　For　it　is　the　nature　of　the　whole　to　move　to　the　point　to　which　the　part　naturally　moves。　Since；　then；　it　would　require　a　force　greater　than　itself　to　move　it；　it　must　needs　stay　at　the　centre。　This　view　is　further　supported　by　the　contributions　of　mathematicians　to　astronomy；　since　the　observations　made　as　the　shapes　change　by　which　the　order　of　the　stars　is　determined；　are　fully　accounted　for　on　the　hypothesis　that　the　earth　lies　at　the　centre。　Of　the　position　of　the　earth　and　of　the　manner　of　its　rest　or　movement；　our　discussion　may　here　end。　　　Its　shape　must　necessarily　be　spherical。　For　every　portion　of　earth　has　weight　until　it　reaches　the　centre；　and　the　jostling　of　parts　greater　and　smaller　would　bring　about　not　a　waved　surface；　but　rather　compression　and　convergence　of　part　and　part　until　the　centre　is　reached。　The　process　should　be　conceived　by　supposing　the　earth　to　come　into　being　in　the　way　that　some　of　the　natural　philosophers　describe。　Only　they　attribute　the　downward　movement　to　constraint；　and　it　is　better　to　keep　to　the　truth　and　say　that　the　reason　of　this　motion　is　that　a　thing　which　possesses　weight　is　naturally　endowed　with　a　centripetal　movement。　When　the　mixture；　then；　was　merely　potential；　the　things　that　were　separated　off　moved　similarly　from　every　side　towards　the　centre。　Whether　the　parts　which　came　together　at　the　centre　were　distributed　at　the　extremities　evenly；　or　in　some　other　way；　makes　no　difference。　If；　on　the　one　hand；　there　were　a　similar　movement　from　each　quarter　of　the　extremity　to　the　single　centre；　it　is　obvious　that　the　resulting　mass　would　be　similar　on　every　side。　For　if　an　equal　amount　is　added　on　every　side　the　extremity　of　the　mass　will　be　everywhere　equidistant　from　its　centre；　i。e。　the　figure　will　be　spherical。　But　neither　will　it　in　any　way　affect　the　argument　if　there　is　not　a　similar　accession　of　concurrent　fragments　from　every　side。　For　the　greater　quantity；　finding　a　lesser　in　front　of　it；　must　necessarily　drive　it　on；　both　having　an　impulse　whose　goal　is　the　centre；　and　the　greater　weight　driving　the　lesser　forward　till　this　goal　is　reached。　In　this　we　have　also　the　solution　of　a　possible　difficulty。　The　earth；　it　might　be　argued；　is　at　the　centre　and　spherical　in　shape：　if；　then；　a　weight　many　times　that　of　the　earth　were　added　to　one　hemisphere；　the　centre　of　the　earth　and　of　the　whole　will　no　longer　be　coincident。　So　that　either　the　earth　will　not　stay　still　at　the　centre；　or　if　it　does；　it　will　be　at　rest　without　having　its　centre　at　the　place　to　which　it　is　still　its　nature　to　move。　Such　is　the　difficulty。　A　short　consideration　will　give　us　an　easy　answer；　if　we　first　give　precision　to　our　postulate　that　any　body　endowed　with　weight；　of　whatever　size；　moves　towards　the　centre。　Clearly　it　will　not　stop　when　its　edge　touches　the　centre。　The　greater　quantity　must　prevail　until　the　body's　centre　occupies　the　centre。　For　that　is　the　goal　of　its　impulse。　Now　it　makes　no　difference　whether　we　apply　this　to　a　clod　or　common　fragment　of　earth　or　to　the　earth　as　a　whole。　The　fact　indicated　does　not　depend　upon　degrees　of　size　but　applies　universally　to　everything　that　has　the　centripetal　impulse。　Therefore　earth　in　motion；　whether　in　a　mass　or　in　fragments；　necessarily　continues　to　move　until　it　occupies　the　centre　equally　every　way；　the　less　being　forced　to　equalize　itself　by　the　greater　owing　to　the　forward　drive　of　the　impulse。　　　If　the　earth　was　generated；　then；　it　must　have　been　formed　in　this　way；　and　so　clearly　its　generation　was　spherical；　and　if　it　is　ungenerated　and　has　remained　so　always；　its　character　must　be　that　which　the　initial　generation；　if　it　had　occurred；　would　have　given　it。　But　the　spherical　shape；　necessitated　by　this　argument；　follows　also　from　the　fact　that　the　motions　of　heavy　bodies　always　make　equal　angles；　and　are　not　parallel。　This　would　be　the　natural　form　of　movement　towards　what　is　naturally　spherical。　Either　then　the　earth　is　spherical　or　it　is　at　least　naturally　spherical。　And　it　is　right　to　call　anything　that　which　nature　intends　it　to　be；　and　which　belongs　to　it；　rather　than　that　which　it　is　by　constraint　and　contrary　to　nature。　The　evidence　of　the　senses　further　corroborates　this。　How　else　would　eclipses　of　the　moon　show　segments　shaped　as　we　see　them？　As　it　is；　the　shapes　which　the　moon　itself　each　month　shows　are　of　every　kind　straight；　gibbous；　and　concave…but　in　eclipses　the　outline　is　always　curved：　and；　since　it　is　the　interposition　of　the　earth　that　makes　the　eclipse；　the　form　of　this　line　will　be　caused　by　the　form　of　the　earth's　surface；　which　is　therefore　spherical。　Again；　our　observations　of　the　stars　make　it　evident；　not　only　that　the　earth　is　circular；　but　also　that　it　is　a　circle　of　no　great　size。　For　quite　a　small　change　of　position　to　south　or　north　causes　a　manifest　alteration　of　the　horizon。　There　is　much　change；　I　mean；　in　the　stars　which　are　overhead；　and　the　stars　seen　are　different；　as　one　moves　northward　or　southward。　Indeed　there　are　some　stars　seen　in　Egypt　and　in　the　neighbourhood　of　Cyprus　which　are　not　seen　in　the　northerly　regions；　and　stars；　which　in　the　north　are　never　beyond　the　range　of　observation；　in　those　regions　rise　and　set。　All　of　which　goes　to　show　not　only　that　the　earth　is　circular　in　shape；　but　also　that　it　is　a　sphere　of　no　great　size：　for　otherwise　the　effect　of　so　slight　a　change　of　place　would　not　be　quickly　apparent。　Hence　one　should　not　be　too　sure　of　the　incredibility　of　the　view　of　those　who　conceive　that　there　is　continuity　between　the　parts　about　the　pillars　of　Hercules　and　the　parts　about　India；　and　that　in　this　way　the　ocean　is　one。　As　further　evidence　in　favour　of　this　they　quote　the　case　of　elephants；　a　species　occurring　in　each　of　these　extreme　regions；　suggesting　that　the　common　characteristic　of　these　extremes　is　explained　by　their　continuity。　Also；　those　mathematicians　who　try　to　calculate　the　size　of　the　earth's　circumference　arrive　at　the　figure　400；000　stades。　This　indicates　not　only　that　the　earth's　mass　is　spherical　in　shape；　but　also　that　as　compared　with　the　stars　it　is　not　of　great　size。

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Book　III　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1

　　WE　have　already　discussed　the　first　heaven　and　its　parts；　the　moving　stars　within　it；　the　matter　of　which　these　are　composed　and　their　bodily　constitution；　and　we　have　also　shown　that　they　are　ungenerated　and　indestructible。　Now　things　that　we　call　natural　are　either　substances　or　functions　and　attributes　of　substances。　As　substances　I　class　the　simple　bodies…fire；　earth；　and　the　other　terms　of　the　series…and　all　things　composed　of　them；　for　example；　the　heaven　as　a　whole　and　its　parts；　animals；　again；　and　plants　and　their　parts。　By　attributes　and　functions　I　mean　the　movements　of　these　and　of　all　other　things　in　which　they　have　power　in　themselves　to　cause　movement；　and　also　their　alterations　and　reciprocal　transformations。　It　is　obvious；　then；　that　the　greater　part　of　the　inquiry　into　nature　concerns　bodies：　for　a　natural　substance　is　either　a　body　or　a　thing　which　cannot　come　into　existence　without　body　and　magnitude。　This　appears　plainly　from　an　analysis　of　the　character　of　natural　things；　and　equally　from　an　inspection　of　the　instances　of　inquiry　into　nature。　Since；　then；　we　have　spoken　of　the　primary　element；　of　its　bodily　constitution；　and　of　its　freedom　from　destruction　and　generation；　it　remains　to　speak　of　the　other　two。　In　speaking　of　them　we　shall　be　obliged　also　to　inquire　into　generation　and　destruction。　For　if　there　is　generation　anywhere；　it　must　be　in　these　elements　and　things　composed　of　them。　　　This　is　indeed　the　first　question　we　have　to　ask：　is　generation　a　fact　or　not？　Earlier　speculation　was　at　variance　both　with　itself　and　with　the　views　here　put　forward　as　to　the　true　answer　to　this　question。　Some　removed　generation　and　destruction　from　the　world　altogether。　Nothing　that　is；　they　said；　is　generated　or　destroyed；　and　our　conviction　to　the　contrary　is　an　illusion。　So　maintained　the　school　of　Melissus　and　Parmenides。　But　however　excellent　their　theories　may　otherwise　be；　anyhow　they　cannot　be　held　to　speak　as　students　of　nature。　There　may　be　things　not　subject　to　generation　or　any　kind　of　movement；　but　if　so　they　belong　to　another　and　a　higher　inquiry　than　the　study　of　nature。　They；　however；　had　no　idea　of　any　form　of　being　other　than　the　substance　of　things　perceived；　and　when　they　saw；　what　no　one　previously　had　seen；　that　there　could　be　no　knowledge　or　wisdom　without　some　such　unchanging　entities；　they　naturally　transferred　what　was　true　of　them　to　things　perceived。　Others；　perhaps　intentionally；　maintain　precisely　the　contrary　opinion　to　this。　It　has　been　asserted　that　everything　in　the　world　was　subject　to　generation　and　nothing　was　ungenerated；　but　that　after　being　generated　some　things　remained　indestructible　while　the　rest　were　again　destroyed。　This　had　been　asserted　in　the　first　instance　by　Hesiod　and　his　followers；　but　afterwards　outside　his　circle　by　the　earliest　natural　philosophers。　But　what　these　thinkers　maintained　was　that　all　else　has　been　generated　and；　as　they　said；　'is　flowing　away；　no